In liquid crystal display (LCD) systems, a liquid crystal cell is typically situated between a polarizer and an analyzer. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal material, which can be altered by the application of a voltage across the cell. The altered light then enters the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, and electronic games for which light-weight, low-power consumption and long-operating life are important features. Also, replacement of the CRT television (CRT-TV) with LCD-TV is rapidly progressing. LCD modes such as In-Plane Switching (IPS) are known to offer high contrast, high speed response and good color reproduction, making LCD systems suitable for TV applications.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays. The primary factor limiting the contrast of an LCD system is the propensity for light to “leak” through liquid crystal elements or cells which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades contrast but also causes color or hue shifts with an associated degradation of color reproduction. This is a common problem with most display modes. However, in the IPS mode LCD, the liquid crystal optic axis changes its direction while remaining in the plane of the liquid crystal cell. This results in better viewing angle characteristics than those associated with other more conventional modes such as the Twisted Nematic (TN) mode.
One of the factors that permits light leakage in the dark state of the LCD is the viewing angle dependence of the crossed polarizes. Here “cross polarizers” shall be understood as a pair of two polarizers with their transmission axes (or equivalently, absorption axes) forming an angle of 90±5°. The polarizers are absorptive dichroic type commonly used for LCD display applications. Very little light can go through crossed polarizers provided the light is impinged in the direction normal to the plane of the crossed polarizers. However, when the light propagation direction deviates from the normal, there occurs a significant amount of light leakage with the maximum leakage occurring at a large polar viewing angle and 45 degrees of azimuthal viewing angle relative to the transmission axis of the polarizer. This is because the effective angle between the transmission axes of the polarizers deviates from 90°.
One way to reduce the light leakage through two crossed polarizers is to insert a compensator between them. The compensators used for polarizers are nominally a combination of an A-plate and a C-plate, as suggested by Chen et al. (“Optimum film compensation modes for TN and VA LCDs”, SID 98 Digest, pp. 315–318 (1998)).
U.S. Pat. No. 6,606,193 discloses a compensator using two biaxial films having 0.15≦Nz≦0.35 and 0.65≦Nz≦0.85, respectively, where Nz is defined as (nx−nz)/(nx−ny) with a relation nx>ny. Here, x and y lie in the plane of the layers, and z lies in the plane normal to the layers. Thus, indices of refraction satisfy nx>nz>ny. The two biaxial films are placed between the pair of crossed polarizers of the LCD. The fast axes (the y direction having index of refraction ny) of both biaxial films are placed parallel to the absorption axis of one of the polarizers. A similar technique is disclosed in Japanese patent publication JP2001-350022, where two biaxial films are used to suppress spectrum dependence of light leakage.
In the afore-mentioned references using compensation films, the aim is to reduce the viewing angle dependence of the effective angle formed by the transmission (or absorption) axis of the crossed polarizers. These techniques can be used to increase the contrast ratio of, for example, the IPS mode liquid crystal display.
Saitoh et al. (SID digest 1998 page 706–709) proposed viewing angle improvement in the IPS mode liquid crystal display (IPS-LCD). A single biaxial compensation film was used in combination with the IPS mode liquid crystal cell and a pair of polarizers. The direction of slow axis of the biaxial compensation film is parallel to the direction of transmission axis of the polarizer and the azimuthal direction of the liquid crystal optic axis of the IPS mode liquid crystal cell in the OFF state. The transmission axis of the other polarizer is perpendicular to the transmission axis of the other polarizer. The configuration reduces the light leakage through the crossed polarizers and thereby increases the contrast ratio of the IPS-LCD.
While compensation film techniques have been suggested for reducing light leakage through the crossed polarizers or combination of crossed polarizer and liquid crystal cells (e.g., IPS mode liquid crystal cells), conventional techniques fail to provide a low-cost and/or simple method of providing such compensation films. For Chen's method to work, the sign of retardation of the A and C plate must have the same sign. U.S. published patent application US 2004002750, filed by the current inventors and incorporated herein by reference, discloses a manufacturing process for a negative C plate. However, there is no straightforward manner to obtain a negative A plate with sufficient phase retardation. Thus, the combination of negative A and C plates for compensation is not plausible. While a positive A plate is widely available, a positive C plate is not. A positive C plate can be made from uniformly and perpendicularly aligned liquid crystal polymer. Polymerizable liquid crystal, such as the one disclosed in U.S. Pat. No. 6,261,649 gives perpendicular alignment. However, liquid crystal polymers are high cost compounds and creating a uniform alignment of liquid crystals poses significant obstacle in large-scale manufacturing. Often non-uniform alignment occurs even in a small size which results in a hazy appearance. Such haze decreases the optical transmission of the compensation film. Polymerizable liquid crystal also requires a photo-polymerization process in order to freeze the perpendicular alignment of the liquid crystal polymer, adding extra processes and cost.
Use of biaxial films also poses problems. Biaxial plates used in both U.S. Pat. No. 6,606,193 and JP2001-350022 have the property nx>nz>ny. Films with such optical property cannot be made without difficulty. Standard manufacturing processes (e.g., stretching of the polymer film) gives films with nx>ny≧nz. Several methods have been suggested to obtain increased nz so that the film can have nx>nz>ny. U.S. Pat. No. 6,606,193 teaches a process in which heat-shrinkable base film is bonded to the secondary film. By heating such an article, the base film shrinks with heat resulting in a higher nz in the secondary film. Another method is the alignment of the polymer segment by an applied electric field. The applied field aligns the polymer segments in the film normal direction, and thus the procedure achieves a higher nz value. Often, however, subsequent stretch is necessary to control all three indices of refraction, nx, ny and nz to their desired values, and careful stretching process is necessary to maintain the relation nx>nz>ny in the final films. Without such fine-tuned stretching, only films having nx>ny≧nz are obtained. Such a fine-tuned process is not suitable for mass manufacturing and may jeopardize the manufacturing repeatability of optical properties of the film.
Thus, problems arise in attempts to provide a multilayer optical compensation film that can be used to prevent light leakage through an LCD in the dark or “black” state.